Steels having a very favorable yield strength/resistance ratio during forming operations have been developed.
They have a very high consolidation capacity, which results in good distribution of deformations in the event of a collision and a much higher yield strength of the part after forming. Thus, it is possible to produce complex parts as with conventional steels, but with higher mechanical properties, which results in reduced thickness that still meets the same functional specifications. These steels are thus an effective response to demands for vehicle weight reduction and safety.
In particular, steels wherein the structure comprises martensite, and potentially bainite, within a ferritic matrix, have undergone extensive development because they combine high strength with high deformation potential.
Recent demands for lighter weight and lower energy consumption have increased the demand for very high-strength steels wherein the mechanical strength Rm is greater than 1180 MPa.
In addition to this high strength level, these steels must have good ductility, weldability, and coatability, in particular good suitability for continuous hot dip galvanizing.
These steels must also have high yield strength and elongation at break as well as good formability.
Indeed, some automotive parts are manufactured by shaping operations that combine various deformation methods. Certain microstructural characteristics of steel can be well suited to one deformation method, but not to another. Certain portions of the parts must have high elongation resistance and/or good bendability and/or stretch flangeability, in particular for shaping curved edges.
This stretch flangeability is evaluated by determining a hole expansion ratio, denoted Ac %. This ratio measures the ability of the steel to expand during cold stamping and therefore provides an assessment of the formability for this deformation method.
The hole expansion ratio can be evaluated as follows: after making a hole by cutting a hole into a metal sheet, a tapered tool is used to expand the edges of this hole. It is during this operation that early damage can be observed near the edges of the hole during expansion, with this damage starting on second phase particles or at the interfaces between the various microstructural components of the steel.
According to documents US 2012/0312433 A1 and US 2012/132327 A1, steels in which the mechanical strength Rm is greater than 1180 MPa are known. However, this mechanical strength is achieved at the expense of formability and weldability.
Furthermore, according to documents US 2013/0209833 A1, US 2011/0048589 A1, US 2011/01683000 A1 and WO 2013/144376 A1, steels having a high mechanical strength exceeding 1000 MPa are known, but do not simultaneously have satisfactory formability and weldability.